1. Field of the Invention
The present invention relates to the field of producing materials for making fuel for nuclear power stations and particularly to a method for the production of low enriched uranium hexafluoride from uranium hexafluoride highly enriched in uranium-235.
The present invention can be utilized for producing low enriched uranium hexafluoride which serves as a starting material for making fuel for nuclear power stations.
2. Description of the Prior Art
In the disclosure of this invention, terminology accepted in world practice is used to describe various degrees of uranium enrichment, namely, highly enriched uranium is uranium enriched to 20% or more by weight in uranium-235; low enriched uranium is uranium enriched up to 5% by weight in uranium-235.
A method of producing low enriched uranium hexafluoride from highly enriched metallic uranium is known (see, for instance, Materials of International Enrichment Conference, June, 1993, Washington).
According to this method, metallic uranium is oxidized and dissolved in nitric acid to produce a liquid uranyl nitrate hexahydrate (UNH). The highly enriched UNH is mixed in a solution with natural or low enriched UNH to produce a homogeneous mixture of low enriched UNH. Impurities are removed using a standard solvent extraction process. The resulting purified low enriched uranium product can be shipped to reactor fuel fabricators in specially licensed containers.
This method does not make it possible to obtain final product which can be immediately utilized for fuel production. It is necessary to conduct one more reaction to convert UNH into uranium hexafluoride UF.sub.6. It should be noted that metallic uranium is converted into a solution and the method is carried out in a liquid phase. From the point of view of nuclear safety, it is more dangerous to carry out the process in the liquid phase. Nitric acid is used, which has a negative effect on the environment, since nitric oxides are released in the process of dissolving the uranium oxides. The nitrates get into the air and soil.
Impurities are removed by the method of extraction, wherein inflammable extractants and solvents are used. The purification process substantially lengthens the technological process.
Furthermore, dosing errors inevitably appear during the dosing of liquid reaction components, which errors substantially exceed errors occurring during the dosing of gaseous components.
This method presupposes utilization of an aqueous technological process at all stages of processing highly enriched uranium, which is undesirable since liquid radioactive wastes are produced and a retardant of neutrons (hydrogen and carbon), is introduced, and also because of the probability of a spontaneous nuclear chain reaction.
The method most similar to the claimed method is the method for the production of low enriched uranium hexafluoride from a weapon grade highly enriched metallic uranium (see, for instance, Materials of the International Enrichment Conference, June, 1993, Washington), wherein a highly enriched metallic uranium is oxidized and the oxide of highly enriched uranium is mixed with a natural uranium oxide. After that, the resulting oxide mix is fluorinated and the enriched uranium hexafluoride is purified by means of dual stage distillation. Low enriched uranium hexafluoride is produced by mixing the purified uranium hexafluoride with natural uranium hexafluoride enriched 5% by weight. The low enriched uranium hexafluoride obtained is placed in the containers "30B".
When carrying out this method the danger exists that a nuclear chain reaction might occur. That is why this method includes multiple dilution of the starting material, increasing thereby many times the amount of oxides which need to be converted to uranium fluorides.
Preliminary mixing of highly enriched uranium oxides with natural uranium oxides is necessary because of nuclear safety requirements; highly enriched uranium must not be purified in a distillation apparatus since distillation apparatus designs do not possess nuclear safe configuration.
A large quantity of the product is fed for purification, and therefore, the energy consumption is large. The components are dosed in comparatively small volumes, the accuracy thereof being insufficient, and this affects the quality of the resultant product.
This process provides for distillation purification of the product. This is a complicated process carried out in a liquid phase at a pressure of 7 atm in an apparatus with a large working volume and high productivity. Whereupon the danger of an ecological catastrophe exists.